Two U.S. astronauts are on their third and final spacewalk of their visit to the space station to complete work on a new air conditioning system for the expanding outpost.

Astronauts from the visiting shuttle Atlantis, David Wolf and Piers Sellers, are connecting the last set of coolant tubes running through three large external heat radiators.

The radiators are attached to a 15-meter aluminum girder the shuttle team attached to the station last week. It is the second of 11 segments that will form a 100-meter backbone for cooling, power, and data relay equipment for future research laboratories.

Before the spacewalk began, U.S. ground controllers in Houston opened one of the folded radiators to test the remote deployment system, although it will not be needed for operations until next year.

The spacewalkers are also performing minor maintenance tasks on the new girder before Atlantis departs the station Wednesday.